This invention relates to a process for the preparation of hydrolytically stable cationic coating compositions, to the coating compositions prepared by said process and to the use thereof, particularly to their use as aqueous thermosetting coating compositions for application via electro deposition.
In the copending British Patent Application, application No. 8616162, water-thinnable, hydrolytically stable, thermosetting cationic coating compositions have been described, comprising a blend of a carboxylated crosslinking compound and an amine group-containing resin binder, being the reaction product of ammonia and at least a specific type of polyglycidyl ether. Said cationic coating compositions are suited for coating the interior of beer and beverage cans, especially for such applications where the taste performance requirements are important.
In order to achieve the combination of performance requirements of good taste performance and good mechanical and chemical resistance, it has been found useful to employ a carboxylated crosslinking compound in the cationic coating composition in place of conventional crosslinking compunds such as non-carboxylated melamine formaldehyde resins. It is believed that the presence of the carboxy groups in the crosslinking compound has a catalytic effect on the crosslinking reaction during stoving.
A disadvantage associated with the use of carboxylated crosslinking compounds such as carboxylated melamine formaldehyde resins, is that they are not as widely available as their non-carboxylated counterparts, and thus the applicability of these amino group-containing binders is limited. Moreover, if the performance of the ultimate coating requires the concentration of the carboxy groups as total binder to be adjusted, this can only be done by simultaneously changing the ratio of the amino group-containing binder and the crosslinking compound. This change in binder composition would be reflected in the balance of performance properties, especially the mechanical performance properties.
it is therefore an object of the invention to develop cationic coating compositions which do not have the disadvantages inherent to the use of the carboxylated crosslinking compounds but have the high level of performance properties which such carboxylated crosslinking compounds impart to cationic coating compositions.